When irradiated nuclear reactor fuel elements are reprocessed, highly active waste is obtained in the form of highly active liquid concentrates of fission products. These liquid concentrates are solidified by appropriate vitrification processes. Glass-forming materials are added and the radioactive materials are melted to glass. The radioactive glass melt is placed in metal vessels made of special steel, so-called molds. After cooling, solidification and possibly quite long surface storage, it is intended that the glass-filled steel molds be taken to the ultimate storage location.
The melting of the glass-forming materials with the radioactive substances is usually carried out in a ceramic melting furnace directly heated by electricity. In the furnace the calcined fission product is continuously fused into the bath of glass melt. Metal vessels are then filled at intervals with the glass containing the radioactive substances.
Essentially three methods are known for filling the metal vessels from the glass melting furnace, namely: the bottom discharge system; the overflow system; and, the suction method.
The bottom discharge system basically includes an opening in the bottom of the furnace in which the glass can either be frozen up by cooling or melted by heating. If the glass in the bottom opening is melted, the glass melt running out fills a metal vessel standing under the furnace.
With the overflow system the melt is preferably discharged via a second chamber of the melting furnace having a port in the side wall. The second chamber communicates with the main chamber at the bottom of the furnace. When a given degree of fullness is exceeded, the glass runs out of the port in the side wall and through a horizontal overflow pipe into the metal vessel.
In the suction method, a partial vacuum is established in the metal vessel and the vessel is sealed in a vacuum-tight manner. After a sealed suction tube mounted on the metal vessel dips into the glass melt from above and after the seal in the suction tube melts open, the partial vacuum in the metal vessel causes the glass melt to be drawn by suction into the closed metal vessel.
The suction method has important advantages. The quantity of glass melt drawn in is determined by the partial vacuum in the vessel. It is no longer possible to overfill the vessel. Furthermore, sediments located in the furnace can be drawn up with the glass melt by utilizing suction.
A process of this type, where the glass is removed from a ceramic melting furnace by the suction method, is known from published German patent application DE-OS No. 29 27 795. In this process, the suction tube projects through a suction port into the glass melt in the furnace and is joined to the metal vessel in a vacuum-tight manner until the filling process is completed. Thereafter, the suction tube is separated from the vessel and broken up. The fragments of the suction tube are placed in a further empty vessel.
Published German patent application DE-OS No. 29 27 795 discloses that the suction port of the glass melting furnace should be equipped with a yielding closure. The closure should preferably be in the form of an annular lamellar diaphragm biased in the closing direction. The flexible closure of the suction port of the furnace has been found to have a short life because of the heavy load.
A further method of removing glass from a ceramic melting furnace with suction is known from German Patent 30 22 387. In this method, the suction tube, joined to the bottom of a metal vessel to be filled, is sealed when the vessel has been filled, and is pushed axially inside the vessel through an opening in the bottom thereof. The opening is subsequently closed by inserting a vessel sealing cover therein. The cover is inserted in a separate method step. The metal vessel is transported away from the glass melt furnace to a guide device and is placed in the guide device which prevents any canting of the vessel with the suction tube projecting from the bottom thereof. The sealing cover is pre-positioned beneath the metal vessel with the inner surface thereof facing upwardly. The vessel is lowered by gravity. The mouth of the suction tube touches down on the cover. The weight of the glass-filled metal vessel causes the suction tube to be severed at predetermined breaking locations provided in the bottom of the vessel and to be pushed inside the vessel. The vessel is lowered over the sealing cover. The opening in the bottom of the vessel through which the tube is pushed into the vessel interior is closed by this sealing cover. The sealing cover is placed on the vessel by means of a shrink fit.